A strong argument that the novel human retrovirus XMRV is not a laboratory contaminant is the the finding that viral DNA is integrated in chromosomal DNA of prostate tumors. Why does this result constitute such strong proof of viral infection?
Establishment of an integrated copy of the viral genome – the provirus – is a critical step in the life cycle of retroviruses. Proviral DNA is transcribed by cellular RNA polymerase II to produce the viral RNA genome and the mRNAs required to complete the replication cycle. Without proviral DNA, retroviral replication cannot proceed.